Interactions Between Fluence Variations and Material Properties Data for Large Surveillance Capsules

Abstract

The Integrated Reactor Vessel Materials Surveillance Program of the Babcock & Wilcox Owners Group 177-fuel assembly reactor vessels uses a number of capsules that are significantly larger than those used in most surveillance programs. The larger cross section poses a problem because the specimens are grouped and cause a significant attenuation of the neutron flux through each set of specimens. In the region containing the Charpy specimens, arranged in a 4 × 4 matrix, the attenuation is approximately 40%. To minimize the attenuation, the specimens were grouped so that each row or stack was made up of specimens from one material. The specimen data were compared with unirradiated data to determine the sensitivity of the material properties to fluence variation. The arrangement of the large compact specimens permits fracture testing to propagate the crack into regions of increased embrittlement. The small amount of material involved in the testing, a result of the short distance the crack is propagated, reduces the importance of fluence attenuation. The results imply that highly accurate fluence values may not be essential to evaluating irradiated mechanical properties data.